The present invention relates to a protective coating for metallic articles and a method of applying a protective coating to a metallic article. The present invention relates in particular to a protective coating for a nickel base superalloy article or a cobalt base superalloy article.
Conventional environmental protective coatings for nickel base superalloys, cobalt base superalloys and iron base superalloys include aluminide coatings, platinum modified aluminide coatings or chromium modified aluminide coatings for high temperature oxidation and Type 1 corrosion resistance.
Conventional environmental protective coatings for nickel base superalloys, cobalt base superalloys and iron base superalloys include silicide modified aluminide coatings or chromised coatings for lower temperature Type 2 corrosion resistance.
Aluminide coatings are generally applied by the well-known pack aluminising, out of pack aluminising or slurry aluminising processes. Platinum coatings are generally applied by electroplating, sputtering, or physical vapour deposition processes. Chromium coatings are generally applied by pack chromising or out of pack vapour chromising. Silicide coatings are generally applied by slurry aluminising.
It has been found that the roots, shanks and internal cooling passages of the turbine blades are suffering corrosion, particularly low chromium nickel base superalloy turbine blades. The roots, shanks and internal cooling passages of the turbine blades suffer from Type 2 corrosion, this is a particular problem at low temperatures, below about 850xc2x0 C. The corrosion may lead to stress cracking of the aerofoils and/or roots of the turbine blades.
In the case of turbine blades, or turbine vanes, for gas turbine engines it is known to provide more than one environmental protective coating if more than one type of oxidation or corrosion is experienced.
Platinum aluminide coatings may be provided on the aerofoils of the turbine blades and chromised coatings may be provided on the shanks, roots and internal cooling passages of the turbine blade to provide environmental protection.
However, it has been found that for some metallic articles, that once the chromised coating has been penetrated by the corrosion, the corrosion of the underlying metallic article occurs at a greater rate than a metallic article without a chromised coating.
Platinum aluminide coatings may be provided on the aerofoils of the turbine blades and silicon aluminide coatings may be provided on the shanks, roots and internal cooling air passages of the turbine blade to provide environmental protection.
However, the silicon aluminide coating provides very good corrosion resistance but the silicon aluminide is brittle, suffers from cracking and is therefore not suitable for use on the roots of the turbine blades.
Additionally any coating for the root of the turbine blade must not impair the fatigue life of the root, shank or other portion of the turbine blade to such an extent that the turbine blade is unusable for practical purposes.
Accordingly the present invention seeks to provide a novel corrosion protective coating for a metallic article and a novel method of applying a corrosion protective coating to a metallic article which reduces, preferably overcomes, the above mentioned problems.
Accordingly the present invention provides a corrosion protective coating for a metallic article, the metallic article having at least one highly stressed region, the metallic article comprising a gamma phase and a gamma prime phase, the corrosion protective coating being arranged on the at least one highly stressed region of the metallic article, the corrosion protective coating comprising a platinum-group metal enriched outer layer on the metallic article, the outer layer of the metallic article predominantly comprising a platinum-group metal enriched gamma phase and a platinum-group metal enriched gamma prime phase.
Preferably the platinum-group metal enriched outer layer has a compressive stress.
Preferably the corrosion protective coating comprises a thin layer of oxide on the platinum-group metal enriched outer layer of the metallic article.
Preferably the platinum-group metal is platinum.
Preferably the platinum-group metal enriched outer layer of the metallic article comprises a controlled amount of silicon, aluminium or chromium.
Preferably the metallic article comprises a nickel base superalloy or a cobalt base superalloy.
Preferably the metallic article comprises a turbine blade or a turbine vane. Preferably the coating is on the root and/or shank of the turbine blade.
The present invention also provides a method of applying a corrosion protective coating to a metallic article, the metallic article having at least one highly stressed region, comprising the steps of:
applying a layer of platinum-group metal to the at least one highly stressed region of the metallic article, the metallic article comprises a gamma phase and a gamma prime phase,
heat treating the metallic article to diffuse the platinum-group metal into the metallic article and thereby create a platinum-group metal enriched outer layer on the metallic article, the heat treatment being carried out at a temperature and for a time sufficient such that the platinum-group metal enriched outer layer of the metallic article predominantly comprises a platinum-group metal enriched gamma phase and a platinum-group metal enriched gamma prime phase.
Preferably a compressive stress is produced in the platinum-group metal enriched outer layer on the metallic article.
Preferably the method comprises forming a thin layer of oxide on the platinum-group metal enriched outer layer of the metallic article.
Preferably the heat treatment is carried out at a temperature of 1000xc2x0 C. to 1200xc2x0 C. More preferably the heat treatment is carried out at a temperature of 1100xc2x0 C. to 1200xc2x0 C.
Preferably the heat treatment is carried out for up to 6 hours. More preferably the heat treatment is carried out for up to 2 hours.
Preferably the platinum-group metal is applied by electroplating, physical vapour deposition, chemical vapour deposition, plasma assisted chemical vapour deposition.
Preferably the platinum-group metal is platinum.
Preferably the thickness of the layer of platinum-group metal as applied before heat treatment being between 2 micrometers and 12 micrometers. More preferably the thickness of the layer of platinum-group metal as applied before heat treatment is in the range 6 to 8 micrometers.
Preferably the thin adherent layer of oxide is formed by heating the platinum-group metal enriched outer layer in an oxygen containing atmosphere.
Preferably a controlled amount of silicon, aluminium or chromium is applied with or to the layer of platinum-group metal.
Preferably the metallic article comprises a nickel base superalloy or a cobalt base superalloy.
Preferably the metallic article comprises a turbine blade or a turbine vane. Preferably the coating is applied to the root and/or shank of the turbine blade.
Preferably the producing of the compressive stress in the platinum-group metal enriched outer layer on the metallic article comprises shot peening, laser shock peening or other suitable methods of peening.
Preferably the shot peening comprises directing metal particles or ceramic particles onto the platinum-group metal enriched outer layer on the metallic article.
The present invention will be more fully described by way of example with reference to the accompanying drawings in which: